After this commit, a value of type 'expr' cannot be a reference to nullptr.
This commit also fixes several bugs due to the use of 'null' expressions.
TODO: do the same for kernel objects, sexprs, etc.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
rewrite_* functions take the rewriting results of the sub-components and
construct the rewriting result for the main component.
For instance, rewrite_app function takes env, ctx, and the value v s.t.
v = (e_0 e_1 ... e_n)
and the rewriting results for e_i's as a vector(buffer)
(e'_0, pf_0 -- proof of e_0 = e'_0)
(e'_1, pf_1 -- proof of e_1 = e'_1)
...
(e'_n, pf_n -- proof of e_n = e'_n).
Then rewrite_app function construct the new v'
v' = (e'_0 e'_1 ... e'_n)
and the proof of v = v' which is constructed with pf_i's.
These functions are used in the component rewriters such as app_RW and
let_type_RW, as well as more complicated rewriters such as depth
rewriter.
The following call sequence is possible:
C++ -> Lua -> C++ -> Lua -> C++
The first block of C++ is the Lean main function.
The main function invokes the Lua interpreter.
The Lua interpreter invokes a C++ Lean API.
Then the Lean API invokes a callback implemented in Lua.
The Lua callback invokes another Lean API.
Now, suppose the Lean API throws an exception.
We want the C++ exception to propagate over the mixed C++/Lua call stack.
We use the clone/rethrow exception idiom to achieve this goal.
Before this commit, the C++ exceptions were converted into strings
using the method what(), and then they were propagated over the Lua
stack using lua_error. A lua_error was then converted into a lua_exception when going back to C++.
This solution was very unsatisfactory, since all C++ exceptions were being converted into a lua_exception, and consequently the structure of the exception was being lost.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
